In general, a thermal recording sheet is obtained by mixing a normally colorless or pale colored dye precursor and a color developer such as a phenolic compound, each dispersed to fine particles and mixed, adding a binder, a filler, a sensitizer, a lubricant, and other additives to form a coating color, and coating the coating color on a substrate such as paper, synthetic paper, films, or plastics, which develops a color by a momentary chemical reaction caused by heating with a thermal head, a hot stamp, a thermal pen, laser light or the like to obtain a recorded image.
Thermal recording sheets are applied in a wide variety of areas such as measuring recorders, terminal printers for computers, facsimiles, automatic ticket venders, and bar code labels. However, with recent diversification of these recording devices and advance towards higher performance, quality requirements for thermal recording sheet have become higher and more difficult to achieve. For example, for high-speed recording, a thermal recording sheet which can provide a high recording density even with a small thermal energy is in demand. On the other hand, in view of storage stability of recording sheet, a thermal recording sheet is required which is superior in light resistance, heat resistance, water resistance, oil resistance, and plasticizer resistance.
Further, with the popularization of plain paper recording system such as electrophotographic or ink-jet systems, the thermal recording system has become often compared with these plain paper recording systems. For this reason, for example, stability of recorded portion or stability of unrecorded portion (background portion or white portion) before and after recording are required to be closer in quality to those of plain paper recording, as in the case of toner recording. Further, the thermal recording sheet is required to have a background color stability to heat of above 100.degree. C because the thermal recording sheet is used as a label for foodstuffs which are subjected to, sterilization at high temperatures, and in cards such as skiing lift tickets which are heat laminated.
As to the background color stability of thermal recording material, for example, Japanese Patent Laid-open Publication (OPI) 04-353490 discloses a thermal recording material containing 3-dibutylamino-7-(o-chloroanilino) fluorane, 4-hydroxydiphenylsulfone compound having a melting point of above 120.degree. C., and a mixture of sodium salt of 2,2'-methylenebis(4,6-di-tert-butylphenyl)phosphate and magnesium silicate having a relatively good background color stability and good stability of recorded image even at a high temperature of about 90.degree. C.
On the other hand, recording on the thermal recording sheet is generally achieved by contacting a thermal head or IC pen as a heating element directly on the thermal recording paper. In this method, a color developing melt or the like tends to adhere to the heating element, resulting in a degraded recording function. Further, a thermal recording method using a thermal head is limited in increasing the density of the heating element, the resolution is typically about 10 dots/mm, and recording of a higher density is difficult. Then, a noncontacting recording method with light is proposed as a method for further improving the resolution without degrading the recording function.
Japanese Patent OPI 58-148776 discloses that thermal recording is possible using a carbon dioxide laser as a recording light source, by converging and scanning the laser light on the thermal recording paper. This recording method requires a high laser output power in spite of the fact that the thermal recording paper absorbs the oscillation wavelength of the carbon dioxide laser. The recording apparatus is impossible to be designed compact partly because of the use of a gas laser, and has a problem in fabrication cost.
Further, since conventional thermal recording paper is hard to absorb light in the visible and near-infrared regions, when a laser having an oscillation wavelength in the visible or near-infrared region, a required heat energy cannot be obtained unless the laser output power is increased to a great extent.
Still further, optical recording materials comprising combinations of conventional thermal recording materials and light absorbent materials are proposed in Japanese OPIs 54-4142, 57-11090, 58-94494, 58-209594, and so on.
Japanese OPI 54-4142 discloses that in a thermal recording medium having a substrate coated thereon with a thermal recording layer mainly comprising a leuco dye, using a metal compound having a lattice defect, the metal compound absorbs light of the visible or infrared region to convert it to heat, thereby enabling thermal recording. Japanese OPI 57-11090 describes an optical recording medium having a recording layer comprising a colorless or pale colored color forming substance, a phenolic substance, and an organic polymer binder, containing therein a benzenedithiol nickel complex as a light absorbent, which allows recording with laser light. Japanese OPI 58-94494 discloses recording medium having a substrate coated thereon with one or more thermal color forming materials, and one or more near-infrared absorbent material comprising a compound having a peak absorption wavelength in the near-infrared region of 0.7 to 31 .mu.m. Japanese OPI 58-209594 discloses an optical recording medium characterized in that at least one set of a near-infrared absorbent material having an absorption wavelength in the near-infrared region of 0 8 to 21 .mu.m and at least one thermal color forming material is coated on a substrate.
The thermal recording medium disclosed in Japanese OPI 04-353490 has a heat stability of background color (hereinafter referred to as "heat stability") that a Macbeth density of the background color is 0.11 after the medium is treated in a hot air dryer at 95.degree. for 5 hours, which is fairly good in stability, but is yet insufficient in terms of heat resistance temperature.
With heat resistance of conventional thermal recording materials using a phenolic color developer, it has been impossible to heat laminate the recording surface or the entire recording medium with a film or the like after thermal recording. On the other hand, with heat resistance of conventional optically recordable thermal recording medium using a phenolic color developer as a recording material, when the unrecorded optically recordable thermal recording surface or the entire recording medium is heat laminated with a film or the like, or the recorded surface or the entire recording medium after thermal or optical recording is heat laminated with a film or the like, the overall surface develops a color, and is thus impracticable.
Therefore, a primary object of the present invention is to provide a thermal recording medium capable of thermal recording or optical recording which has a heat resistance to an extent that can be heat laminated.